In hydraulic presses or forging machines having a hydraulic drive that operate with so-called crank-type driving mechanisms, such as is known for a forging machine e.g. from German patent 36 07 737, a single-cylinder crank pump drives a press cylinder using a pressure multiplying effect that corresponds to the ratio of the cylinder areas and a sinusoidal stroke movement the amplitude of which is in inverse proportion to the cylinder areas. The stroke frequency is determined by the speed of rotation of the pump drive. A quantity of fluid moved back and forth by the crank-type driving mechanism acts on the press cylinder so that this passes on to the press piston a pendulum or up-and-down movement in time with the crank-type driving mechanism. These drives enable extraordinarily high stroke rates to be achieved.
From DE-AS 12 54 433 a means of controlling the stroke movement of an upper tool holder of an electrohydraulic hammer forging press having a crank-type drive is known that makes it possible to shift the position of the working stroke upwards and downwards to adapt it to the predetermined size of the body being forged by letting out part of the operating pressure medium or supplying it from an accumulator and thus reducing or increasing the volume of fluid that flows back and forth. In order to supply or eject pressure medium during the working stroke in the amount required in the event of a change in forging resistance a stroke position overlapping the body being forged is used and, by means of an additional inlet and outlet valve, the working stroke is interrupted on reaching the forging body by draining out pressure medium and the starting point of the working stroke is reset by letting in the same amount of pressure medium shortly before the upper turning point.
To enable the force of the press to be applied at all times during the whole stroke of the cylinder, expensive servo valves are required to introduce the pressure medium fluid into or remove it from the connecting line between the pump or pumps of the crank-type drive and the press cylinder so as to displace the stroke position of the press cylinder. Apart from the cost involved, the additional quantity of pressure medium can only be introduced in the pressureless phase during the return movement of the press, since otherwise large amounts of power would be needed, so that only correspondingly small changes in stroke are possible. However, for working procedures that require relatively long working strokes such small changes in stroke are inadequate.